The present invention relates to a substrate for device fabrication, the substrate having a monocrystal Ge thin layer for forming field effect transistors of high-performance thereon, a semiconductor device using this substrate, and a method for manufacturing the substrate.
Conventionally, a method for increasing a drive current per a unit gate length by shortening a gate length of an individual transistor and thinning a gate insulation layer is adopted for realizing high-performance/high function of CMOS circuit device. By this method, the size of a transistor to provide a necessary drive current is decreased. This makes it possible to realize a high integration, and to lower a drive voltage, resulting in decreasing a power consumption per a unit element.
However, improvement of performance required in late years increases a technical barrier to be solved for the purpose of decreasing a gate length. It is effective to use channel materials of high mobility in order to relax the circumstances. Ge is an influential candidate for the channel materials. Ge has higher mobility than Si with respect to electrons and holes. It is known that hole mobility largely increases by giving a compressive strain to Ge. In a bulk semi-conductor, hole mobility is low in comparison with electron mobility. Therefore, increase of hole mobility contributes to higher performance of a circuitry.
There is a problem that a parasitic capacitance of source and drain junctions disturbs a transistor operation which is caused by micronization of a transistor. A fully-depleted type device structure wherein a buried insulator layer is formed under a semi-conductor thin channel layer is considered in order to avoid this problem. The film thickness of the semi-conductor thin channel layer in this case is not more than about 6 nm with respect to a transistor of a gate length 25 nm, for example. If a channel is formed by a strained Ge thin film on a buried insulating layer combining the feature of a strained Ge channel and that of a fully-depleted type device structure, it is possible to fabricate a high performance transistor. However, an on-insulating film laminating strain Ge thin layer having these both features is not realized under the present circumstances.
In a document “T. Tezuka, N. Sugiyama, S. Takahi, Appl. Phys. Lett. 79, p1798 (2001)”, the inventors of the present invention proposes the Ge-condensation by oxidation method to make Ge composition in SiGe increase by oxidizing a monocrystal Si layer formed on an insulating film on a supporting substrate and a monocrystal SiGe layer containing Ge composition of about 10% which is formed on the Si layer. However, this method is a method for manufacturing a lattice-relaxed SiGe layer of high Ge composition as a substrate for a strained Si layer, unlike a method for forming a strained Ge thin layer. Further, this method does not consider thinning the film thickness of the Ge layer.
The substrate having a strained Ge thin layer on an insulating film is expected as a substrate used for making a field effect transistor with high mobility. However, a technique to form a strained Ge thin layer of extremely thin film thickness on an insulating film has not yet been realized.
The present invention is to provide a substrate for device fabrication having an extremely thin Ge layer on an insulating film.